The intrinsic oscilloscope risetime must be known for correct interpretation of oscilloscope measurements. For this purpose PTB has developed an optoelectronic method to determine the risetime of 50 GHz oscilloscopes. The method utilizes optical femtosecond pulses and allows traceable measurements of oscilloscope risetimes of only a few picoseconds.

Photoconductive switch and laser beams (coloured in red) for the generation and measurement of ultrashort voltage pulses.

Broad-band 50 GHz oscilloscopes are indispensable tools in the development of ultrafast electronic circuits in data processing and communication technology. These oscilloscopes have very short intrinsic risetimes and allow the visualization of ultrafast electric signals. Nonetheless, the risetime is not zero, and the measurement curves may be distorted. To correct this distortion, the user needs to know the risetime of the oscilloscope. As its determination calls for a measurement method with an even higher temporal resolution, methods of femtosecond optics are used.

To determine the risetime of 50 GHz oscilloscopes, short voltage pulses of approx. 1 ps are generated on a coplanar waveguide by short-circuiting a photoconductive switch with laser pulses of 100 fs. The voltage pulses are coupled into the oscilloscope via a microwave probing tip. An opto-electronic probing method allows one to measure the voltage pulses on the waveguide with a temporal resolution of 300 fs. By means of such measurements it is possible to determine the deformation of the pulses on their way to the oscilloscope and to calculate their shape at the oscilloscope input. Deconvolution of the oscilloscope curve with the known input pulses gives the response characteristics of the oscilloscope, in particular, its risetime.

First measurements on a 50 GHz oscilloscope yielded risetimes in the range of 7 ps with a typical expanded measurement uncertainty of 1,5 ps. Oscilloscopes calibrated by this method allow for reliable measurements in the area of microwave electronics.